The present invention concerns a nozzle for treating web-like materials and by which air, or another equivalent gaseous fluid is blown into contiguity but without contact with the material web that is to be supported and possibly at the same time to be otherwise treated, such, for example, as dried said nozzle having a substantially annular nozzle slit and a carrying surface producing said supporting effect.
Various types of air nozzles are employed to blow in gas in apparatus used on paper manufacturing and conversion machines for contact-free cleaning, drying and stabilizing of the web. The gas blown by the aid of said nozzles is conducted onto one or both sides of the web, whereafter the gas is drawn off to be reused before the next nozzle.
The said nozzles of in the prior art may be divided into two groups: overpressure nozzles and subatmospheric pressure nozzles. The operation of the former is based on the so-called air cushion principle, whereby the air jet causes a static over-pressure in the space between the nozzle and the web.
The subatmospheric nozzles include the so-called airfoil nozzles, which attract the web and stabilize its course. The attraction exerted on the web results, as is well known from the presence of a gas flow field parallelling the web and which as a result of its flow state gives rise to a static subatmospheric pressure between the web and the supporting surface of the nozzle, the so-called carrying surface. In overpressure as well as subatmospheric pressure nozzles the so-called Coanda phenomenon is often applied in order to direct the air in desired direction.
The overpressure nozzles of the prior art direct sharp air jets against the web. Such a local air jet greatly enhances the heat transfer at the localized areas where the air jet and the web meet, thus giving rise to unequal heat transfer coefficient distribution in the longitudinal direction of the web, and this fact may inflict quality damage on the web that is being treated. Another drawback in the use of overpressure nozzles is that owing to the over-pressure feature they cannot be applied in one-sided treatment of the web.
Regarding the patent literature associated with the present invention, reference is made to the following: U.S. Pat. No. 3,587,177, German publicizing print No. 2.020.430, and Finnish Patent No. 42 522.
It is a characteristic feature in the design of the nozzles disclosed in U.S. Pat. No. 3,587,177 and in the German publicizing print No. 2.020.430 that the nozzle slit opening onto the entrance edge of the nozzles' carrying surface is extended onto a curved flow guiding surface connecting with the front edge of the carrying surface so that the flow can be made to follow the carrying surface. The drawback of these prior art nozzles of prior art is that the gas flow parallel to the web tends to eject drying gas from the preceding suction space that has already cooled, thereby lowering the differential temperature between web and drying gas and thus impairing the heat transfer capacity. In these nozzles the distance of the web from the carrying surface is quite small (2-3 mm), which imposes high dimensional requirements on straightness and smoothness of the drying surface (the carrying surface) composed of nozzles. This introduces structurally strict requirements in the manufacturing of great breadth (over 3 m) nozzles spanning the whole web.
Through the Finnish Patent No. 42 522 already cited, a nozzle is known wherein the air is blown on one side of the web in the form of jets substantially parallelling it and which cause in the breadth direction of the web, discontinuities, as a result of which the heat transfer capacity is non-uniform. For the same reason the stability of the web is not good, and this nozzle cannot be used to handle thin webs, owing to the flutter which is then encountered. It is also impossible to use high blowing rates in this nozzle, and it is not usable for two-sided web treatment.
Reference is further made to the same applicant's earlier Finnish patent application No. 781375, the air-dryer nozzle therein disclosed being characterized in that the nozzle slit is located in the gas flow direction before the plane of the curved guiding surface's entry edge and that the ratio of the nozzle slit width and the radius of curvature of said guiding surface has been so selected with the gas flow velocities encountered, that the gas flow will detach from the curved guiding surface substantially before its trailing edge. The circumstances which were realized in the said Finnish patent application may be applied in connection with the present invention as well, in applicable parts.
The modern, high output paper manufacturing and conversion machines and equipment which utilizes the type of nozzles described immediately above are bulky, space-consuming and expensive. Owing to the poor stabilizing capacity of nozzles known in the prior art, it has previously been impossible to give one-sided treatment to heavy material webs other than in the horizontal plane and with blowing from the underside of the web. This circumstance has contributed to restricting the design freedom of the drying section and to increasing the size of the machine and of the building housing it.